Images may be stored in a variety of formats, including in a magnetic tape format for use in video and in a film format for use in projection. It is often desirable to change from one medium to the other medium. A variety of methods and machines exist for conducting such transfers. To convert from a tape to a film, a beam recorder is used wherein the electronic image stored in the magnetic video tape is converted to a signal in the form of an electron beam which is directed at electron sensitive film. The electron beam scans the film in a line-by-line fashion, generating an image on the film. To convert from a film to a tape a flying spot scanner or telecine is used. A source of light, as for example a laser beam or light from a phosphorous screen, scans the film in a raster or line-by-line fashion. The transmitted light is converted into an electrical signal through known means and stored on a video tape. In each of these types of transfers a continuous motion film drive is used to pull the film through the machine.
It has proved difficult to perform transfers in which the image maintains its reference to an original reference. This problem is particularly pronounced when the images are superimposed. For example, when a title is superimposed against a scenic background, the title may be seen to weave or jump in relationship to the background. This problem becomes more pronounced in high resolution processes or as the number of editing and compositing steps increases. The problem derives from the fact that the positional reference for the film and the tape is not the same. When a film is originally shot in a camera the image is mechanically registered relative to the sprocket holes. The tines of the sprocket engage the sprocket holes to register the film relative to the camera optics. Such a system is mechanically "pin registered". When this same film is transferred to video, as for example in a telecine, the film is no longer machine pin registered but is drawn through the machine by a continuous motion film drive. Mechanical pin registration cannot be used in a telecine operated in real time, at 24 or 30 frames per second, as there is insufficient vertical blanking time in the video system to allow pull down of the film between frames.
Because the film is edge guided during a transfer, film weave may be introduced if the edge of the film varies relative to the sprocket holes. In fact, the film edge tends to vary in distance from the sprocket holes due to manufacturing imperfections and tolerances in the film.
Various mechanical solutions have been attempted. One such solution added a mechanical sprocket drive to the continuous film drive at the point where the picture was scanned. This method introduced small but abrupt speed changes as the sprocket tines entered the sprocket hole. This caused random lines of the resultant video picture to separate during recording, which in turn caused unacceptable random horizontal black lines in the recorded picture. More recent attempted mechanical solutions have been directed to mechanical pin registration. Specifically, the film sprocket holes have been lowered onto stationary register pins by a stepping motor controlled by an auxiliary computer. The film is held steady by the pins during the transfer, and then the gate attachment is released to allow the film to advance. Because of the complicated mechanical nature of this attempted solution the transfer speeds are limited. The typical transfer rate through such a method is four frames per second, approximately six times slower than a real time transfer. Additionally, such mechanical pin registrations are unacceptable if a splice is covering a sprocket hole, or if the film stock has an odd sprocket hole size.
Other mechanical solutions attempted include the mechanical measurement of the position of the sprocket hole during the recording or reading of images on the film, and to use this position measurement to adjust the horizontal deflection of the beam. This method has been attempted without much success.
The problems of mechanical pin registration can be solved with electronic pin registration. As disclosed in U.S. Pat. No. 4,104,680, entitled "Method and Apparatus for Film Weave Correction", film weave may be eliminated in tape to film and film to tape transfers by electronically determining the position of the sprocket hole and positioning the resultant image relative to the sprocket hole. As discussed in the patent for the case of a tape to film transfer, the video signal is converted to an electronic beam, which is directed in a raster scan format to the electron sensitive film. Either the main beam or a secondary beam is used to electronically determine the location of the sprocket hole and to generate a weave correction signal, which in turn correctly positions the image on the film. The solution of electronic weave correction permits real time transfers.
While the electronic weave correction solution of the '680 patent applies equally to tape to film and film to tape transfers, some difficulties have been encountered in conducting transfers in certain instances. For example, when conducting a film to tape transfer in which a clear base stock film is used it has proved difficult to determine the edge position of the sprocket hole. If the film is clear there is little difference in the transmittance of light between the sprocket hole and the clear film. This problem did not arise in connection with tape to film transfers even with a clear or transparent film because the electron beam used in the film recording was stopped by the film. Another problem which has been generally encountered is the application of an erroneous correction signal which may be caused by the detection of a defect in the film or the detection of another object such as a dust particle.